1. Field of the Invention
The present invention relates to a radio frequency modulator of a set-top box modulating a video signal of a base band signal into a frequency signal receivable in a TV set, and more particularly, to a frequency modulator having a compensating function compensating for a delay of a chrominance signal with respect to a luminance signal occurring in demodulating a C/L signal in a TV set, and a set-top-box using the same.
2. Description of the Related Art
Generally, a subscriber signal conversion apparatus is required to receive a cable broadcasting signal or a satellite broadcasting signal or use a next generation two-way multimedia telecommunication service, such as a video-on-demand through a TV set, home shopping, a network game, etc. The subscriber signal conversion apparatus is a home telecommunication terminal coupled to the TV set and is called a set-top-box (STB) representing a box disposed on the TV set (hereinafter called as the STB).
The STB includes a basic function of converting a video signal to a public broadcasting channel signal which is received in the TV set. An apparatus for performing the basic function is called a RF modulator.
FIG. 1 is a diagram showing a connection relationship between a demodulator of a TV set and an RF modulator (high frequency modulator) of a conventional set-top-box (STB). A video signal generated from the STB is demodulated in an RF modulator 1 to a broadcasting channel signal to be transmitted to an input terminal of the TV set. The input video signal is inputted to a demodulator 3 through a tuner 2 and demodulated to a base band video signal in the demodulator 3.
Generally, the video signal includes a luminance signal and a chrominance signal. The luminance signal is pulse signals having a bandwidth of about 4.2 MHz, and the chrominance signal is an analog signal having a frequency of 3.5 MHz. A combination of the luminance signal and the chrominance signal is called a composite video signal. In the video signal having the above luminance signal and the chrominance signal, a delay occurring between the luminance signal and the chrominance signal in a demodulation process is called a C/L delay. The C/L delay represents a delayed period of time of the chrominance signal of 3.58 MHz with respect to the luminance signal having about 400 KHz, and the C/L delay of 170 nanoseconds (nsec) during the demodulation process in the TV set.
The C/L delay of the 170 nsec occurs during a demodulating process of the demodulator 3. A picture quality is lowered due to a delay between the luminance signal and the chrominance signal displayed on a TV screen.
Accordingly, in the video signal transmitted from a broadcast company, the luminance signal is delayed with respect to the chrominance signal by the 170 nsec to compensate for the C/L delay so that the picture quality of the video signal is maintained. That is, the C/L delay of the video signal transmitted from the broadcasting company is −170 nsec.
The STB is required to employ a C/L delay compensation function to output the video signal having the luminance signal delayed with respect to the chrominance signal by 170 nsec.
FIG. 2 is a diagram showing the RF modulator 1 compensating for the C/L delay in the STB shown in FIG. 1. A delay compensation circuit (C/L delay circuit or C/L delay compensation circuit) 4 is coupled between a video signal input terminal and the RF modulator 1 to generate a −170 nsec delay between the luminance signal and the chrominance signal of the composite video signal.
Since the C/L delay circuit 4 is mounted on an outside of the RF modulator 1, a noise component existing on a main board of the STB is introduced into the C/L delay circuit 4 to generate a bad effect on the picture quality. Moreover, since the C/L delay circuit 4 is separately mounted on the main board, it is difficult to minimize the STB in size. Furthermore, a set developing period for developing the STB is extended since a set manufacturer needs to finish reviewing circuit characteristics of the STB when we design the STB.
FIG. 3 is a circuit diagram showing the delay compensating circuit 4 of the STB shown in FIG. 1. The delay compensating circuit 4 includes a delay compensation unit 11a and a notch filter unit 11b to linearly change a phase of the video signal to generate the delay between the luminance signal and the chrominance signal. The delay compensating circuit 4 constitutes a plurality of capacitors C, a plurality of inductors L, and a saw filter to form a sound trap of 4.5 MHz.
Therefore, the conventional delay compensating circuit becomes larger in size due to a circuit compensating for the C/L delay using the inductors L and has problems in integrating the circuit in an IC due to the saw filter used for the sound trap.
In order to solve the above problems, such as an increase of the size of the STB or a load on the manufacturer assembling the STB, the C/L delay circuit has been sold as a separate unit, such as ceramic or a board. An assembling process of the STB may become easier. However, it is difficult to minimize the STB in size since the circuit is separately mounted on the main board of the STB.